This invention relates to a fuel injection system for a spark-ignited piston-type internal-combustion engine and includes a fuel injector by means of which fuel is directly injected into the work chamber of the engine cylinder. As concerns the moment of such injection, essentially two types of operational modes are known.
In the first type of operation, also referred to as a "homogenous operation", the fuel is injected at an early moment, generally during the inflow of fresh air, that is, while the intake valve is open. This mode of operation which ensures a good homogenization of the fuel/air mixture, is particularly expedient during a high-load engine run.
In a second type of operation, also referred to as a "stratified-charge operation", the injection of fuel occurs only after the air intake valve is closed and when the upwardly moving piston arrives in the region of its upper dead center. As a result, the fuel is mixed only with one part of the intake air contained in the cylinder and only in a locally limited manner until the mixture is ignited by the ignition system. Such an operation is preferably used during partial-load conditions and during idling. It is an advantage of this operational mode that the engine may run without throttling the intake air and yet without obtaining, in the vicinity of the ignition device, an excessively lean fuel/air mixture which would jeopardize a reliable ignition.
The above-outlined two operational modes may be performed by various methods for introducing the fuel in the cylinder chamber and for providing a fuel/air mixture. These processes may be subdivided into two categories, namely the "jet-guided" and the "wall-guided" processes.
In the "jet-guided" process the injected fuel jet is aimed directly onto the ignition device. The injected fuel cloud mixes with the combustion air and is ignited by the ignition device. A reliable stratified-charge operation is accordingly ensured only if the ignition device is positioned very close to the fuel injector. Such an arrangement involves the disadvantage that only an extremely small operational point-specific ignition window is available and therefore a coordination (tuning) of the fuel jet expansion for large characteristic field ranges is critical. Consequently, the fuel injectors used in such systems have to be manufactured with high precision; even small tolerance deviations or changes in the injector during an extended operation may lead to disadvantageous boundary conditions for the ignition.
Thus, conditions of a reliable ignition in the stratified-charge operation may be ensured only by means of a precise geometrical arrangement of the ignition device and the fuel jet. Consequently, the known methods belonging to this category are designed without a definite and intensive charge motion. In the homogeneous operation, on the other hand, it is precisely such a charge motion that would be needed for improving the homogenization of the fuel/air mixture. The result is loss of power and an increase of the fuel consumption which, in turn, involves a correspondingly heightened release of pollutants into the atmosphere.
It is a further disadvantage of this process that because of the direct exposure of the ignition device to the fuel jet, an increased wear occurs, resulting in a shortened service life of the ignition devices.
According to the "wall-guided" process, in the stratified-charge operation the fuel jet is deflected towards the ignition device by that wall of the combustion chamber which is formed by the piston base. The resulting intensive charge motion has an enhancing effect. This method avoids a direct impingement of fuel on the spark plugs. Tolerance deviations and the operational condition of the fuel injectors are less critical than in the earlier-discussed jet-guided process. It is, however, a disadvantage of the wall-guided process that the fuel impinges on the combustion chamber wall, particularly on the piston base, during a direct injection into the cylinder chamber so that in certain operational conditions an incomplete combustion occurs which results in an increased emission of uncombusted hydrocarbons and soot. This process has been utilized heretofore with intake-side fuel injectors and is based on the formation of a rolling motion of the charge, which is particularly designed with respect to direction and sense of rotation and which guides the mixture jet, in the direction of injection, via the piston base to the ignition device. This type of charge motion is achieved by providing steep, upstanding intake ports, such as disclosed in European Patent No. 0 558 072. Such an arrangement, however, requires a correspondingly greater structural height of the engine. According to another solution, the desired motional configuration of the charge is obtained by a special form of the intake port or by the geometry at the seating zone of the intake valve, as disclosed in European Patent No. 0 463 613. Such a solution, however, has disadvantageous effects on the quality of flow of the intake system and thus on the full-load operation of the engine. In both cases the injection jet is directed towards a depression of the piston base, so that precisely in case of a stratified-charge operation fuel which is still in a liquid state impinges on the piston base. The mixture which is formed there is then, while in contact with the wall of the piston base, guided to the ignition device.
The intensive charge motion required in the stratified-charge operation in this process is, however, disadvantageous in the homogeneous operation because of the resulting increased combustion noises and increased wall heat losses.